In the gas phase process for production of polyolefins such as polyethylene, a gaseous alkene (e.g., ethylene), hydrogen, co-monomer(s) and other raw materials are converted to solid polyolefin product. Generally, gas phase reactors include a fluidized bed reactor, a compressor, and a cooler (heat exchanger). The reaction is maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. The reactor vessel is normally constructed of carbon steel and rated for operation at pressures up to about 31 bars (or about 3.1 MPa). Catalyst is injected into the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in the external cycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations.
Operation of most reactor systems is critically dependent upon good mixing for uniform reactor conditions, heat removal, and effective catalyst performance. The process must be controllable, and capable of a high production rate. In general, the higher the operating temperature, the greater the capability to achieve high production rate. However, as the operating temperature approaches and exceeds the melting point of the polyolefin product, the particles of polyolefin become tacky and melt. This may cause sheeting in the reactor bed, but also may cause problems in the cycle line.
Typically, the cycle line pulls the gas stream from the upper portion of the reactor bed. Near this region, there has been found to be an accumulation of polymer fines (very fine particles of polymer, e.g., less than 125 US mesh), which then get passed through the cycle line. An interplay of forces results in particles agglomerating with adjacent particles, both in the reactor bed and in the cycle line, which causes sticking together of particles and accumulation of resin in the cycle system and its components such as the cooler. As the cooler becomes more fouled, it results in a progressively decreasing cooling efficiency, which can eventually lead to elevated operating temperatures and failure of the process due to inability to properly cool the reaction. To clear the fouling, the system is shut down. This reactor shut down leads to increased costs associated with the cleaning and with lost production time due to the shut down.
In one specific example of this problem, when only bottom discharge is used from a reactor, polymer fines may be allowed to build up around a cycle gas valve, pipe walls, or cooler in the cycle gas system, which can constrict flow to the point where the entire system may be shutdown to prevent extensive damage. As this occurs, there may also be increases in the cycle cooler and distributor plate pressure drops, and the cycle gas valve and cycle gas compressor may hunt for a set point in which they can create a condition which allows the desired flow through the cycle system, which also results in inefficiencies. In some instances, the amount of build up of particles in the cycle gas system may constrict flow to a point where not enough gas flow can be achieved through the cycle gas system to maintain superficial gas velocity, and this may warrant shutdown of the entire system.
Background references include U.S. Pat. Nos. 5,382,638, 5,428,118, and 5,545,378.
Accordingly, it would be desirable to minimize the presence of particles, for example, that may accumulate in the cycle line, particularly near the cooler, while maximizing production rates.